Prebiotics are non-digestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon. The health benefits of prebiotics have been attributed to their ability to stimulate the growth of beneficial gut bacteria, such as Bifidobacteria and Lactobacilli, and/or their ability to modulate immune responses. Prebiotics are typically oligosaccharides, which are indigestible by humans but can be fermented by gut bacteria. The most common prebiotics include inulin, fructooligosaccharides (FOS), galactooligosaccharides (GOS), xylooligosaccharides (XOS), mannanoligosaccharides (MOS) and resistant starches.
Inulin is a storage polysaccharide found in many plants, including wheat, chicory, garlic and onions. Inulin is composed of chains of fructose molecules linked together by beta(2-1) bonds. Although inulin is not digested by humans, it is broken down by gut bacteria into short-chain fatty acids (SCFAs), which are absorbed and used for energy by the host. In addition to its role in providing energy, SCFAs also have several other beneficial effects on human health, including promoting intestinal integrity, reducing inflammation and modulating immune responses. Fructooligosaccharides (FOS) are short chains of fructose molecules linked together by beta(2-1) bonds. FOS are found naturally in small quantities in a variety of fruits and vegetables but can also be produced commercially from sucrose or starch using enzymes. Like inulin, FOS are not digestible by humans but can be fermented by gut bacteria into SCFAs. GOS consist of galactose units linked together by beta(1-4) bonds. GOS occur naturally in human milk but can also be produced commercially from lactose using enzymes. Xylooligosaccharides (XOS) consist of xylose units linked together by beta(1-4) bonds . Xylose is a sugar found naturally in many plant tissues but can also be produced commercially from glucose using enzymes . Unlike FOS and GOS , X OS cannot be completely metabolized b y gut bacteria an d thus passes through t he digestive system unchanged . However , X OS has been shown t o selectively stimulate th e growth o f Bifidobacteria spp . i n vitro an d thus may confer some health benefits to the host . Resistant starches (RS) refer to starchy carbohydrates that resist digestion i n th e small intestine an d reach th e large intestine essentially intact . RS ar e classified int o 4 subtypes based on their physicochemical properties: raw starch , retrograded starch , chemically modified starch es an d enzymatically modified starches . Raw starche s ar e resistant t o digestion due t o their high amylose content (> 20 % ) or because they form tight crystalline structures that limit enzyme access . Retrograded starch es ar e formed when starchy foods ar e cooked then cooled ; these starche s recrystallize i nto a mor e compact structure tha n raw starch es an d ar e therefore more resistant t o digestion . Chemically modifyin g starch es increases their resistance t o digestion b y altering their physicochemical properties; for example , debranching enzymes ca n increase D -glucan levels i n waxy maize starches , making them more viscous an d less susceptible to degradation b y amylase enzymes . Enzymatically modifying starchy foods with transglycosidases ca n generate new types offiber with unique physico chemical propertie s; for example treatment with alpha -amylase generates oligofructoseenrichedinulins which exhibit increased viscosity comparedt otreatmentwith only alpha -amylase followedbytreatmentwith only transglucosi dateses Finally fermentation ofthesetwo typesoffibercan leadto different end products ; while both fibersarefermentedintothe sameend productsinvitrooligo fructo se enriche dinulins showedhigherend productformationthan did control fibers This difference wasattributedtotheir slowerdegradationrateand higher finalmolecular weightwhichfavors longercolon residence timeand greaterfermentation .The primary benefit associatedwithprebiotic consumptionisstimulationofthegrowthofbeneficialgutbacteria, althoughothermechanismsofactionhavealsobeenproposed. For instance prebiotichavebeenshowntomodulatehostimmuneresponses,, reduceinflammation, protect againstcolorectalcancer,reducebloodlipidlevelsand improveglycemiccontrolin diabetics. Probioticstreatorpreventvaginalinfections, urinarytractinfections,] skin infections,] allergiesandasthma,) inflammatoryboweldiseases(IBD)] irritablebowelsyndrome(IBS)), infectiousdiarrhea,) Helicobacterpyloriinfection,) necrotizingenterocolitis(NEC)) among others.]